1. Field of the Invention
The present invention relates to a three-dimensional (hereinafter referred to as 3D) image acquired apparatus and method for acquiring range image in addition to intensity image by a triangulation method. The invention is aimed at increasing the number of measurement points and improving measurement accuracy by use of a polarizing optical system.
2. Description of the Related Art
A method for measuring the shape of a subject is roughly classified into a passive method (triangulation method, shape from X) and an active method (time of flight method, triangulation method). The different between the passive method and the active method is based on whether some energy is applied on the subject or not. It is generally said that the active method is a measuring method more robust against noise than the passive method because the active method can eliminate ambiguity from measurement.
The triangulation method used in each of the passive and active methods is a geometrical method in which a distance to a measurement point on a subject is obtained on the basis of the length of a base line and angles between the base line and lines connecting opposite ends of the base line to the measurement point on the subject. As the triangulation method used in the active method, there has been proposed a measuring method called “multi-pattern projecting method” in which encoded stripe pattern light with a plurality of stripes is projected as disclosed in “Chu-Song Chen, “Range data acquisition using color structured lighting and stereo vision”, Image and Vision Computing 15, pp. 445-456, 1997”.
FIG. 11 is a configuration diagram showing this measuring method. In FIG. 11, light with a plurality of stripes encoded in accordance with the intensity or color of the light is projected onto a subject 12 by a projector system 11, so that the shape of the subject 12 is measured collectively by two eyes' stereoscopic vision. A reflected image with a plurality of stripes reflected on the subject 12 is captured by a first image input system 13 and a second image input system 14 which are disposed in the left and right of the projector system 11 respectively. Two images acquired by the image input systems 13 and 14 are compared with each other to thereby decide corresponding points on the subject 12. When the corresponding points are decided, the distance to the subject 12 can be obtained by the aforementioned geometrical method.
Natural light (random polarized light) having various polarizing directions as shown in FIG. 12 is used as the projected light. In this case, if a subject high in specular gloss such as a glossy substance is used, a spatial distribution of reflection of light is biased greatly because a measurement surface is deviated largely from perfect diffuse reflection. As a result, the reflected images captured by the left and right image input systems are widely different in intensity value from each other though the reflected striped images are formed from the same striped light. When, for example, the image input systems are located on view points A, B and C respectively, the view point A (in a direction of specular reflection) contains both specular-reflected light high in reflected light intensity and diffuse-reflected light whereas each of the view points B and C contains only diffuse-reflected light. Accordingly, in the method in which striped images acquired by the left and right image input systems are compared with each other so that corresponding points on the subject are decided on the basis of the intensity difference between the striped images, there arises a problem that a great deal of mistaken correspondence occurs or such corresponding points cannot be found.
FIG. 13 is a configuration diagram of an apparatus using polarizing filters in order to remove the specular-reflected light. In this configuration, randomness of the projected light provided as random polarized light is, however, retained in the specular-reflected light observed at the view point A located in a direction of specular reflection. Accordingly, the specular-reflected light cannot be removed, no matter how possibly each polarizing filter is rotated to be adjusted. Hence, in the present circumstances, the aforementioned problem cannot be solved.
Reflected images acquired by the image input systems B and C become substantially equal to each other. If two image input systems are set in this layout, there is some case where mistaken correspondence is reduced. It is however impossible to adapt this configuration to the case where the subject has a plurality of inclinations to bring a limit to the layout of image input systems. When the spread in a spatial distribution of specular-reflected light shifts from the case of a relatively narrow spread shown in FIGS. 12 and 13 (a glossy substance high in specular gloss) to the case of a broad spread (a glossy substance low in specular gloss), mistaken correspondence occurs because part of specular-reflected light is observed in the image input system B. The case where this configuration can be adapted to the reduce of mistaken correspondence is in a very narrow range. That is, this configuration is not realistic.
This example has been described on the case where two image input systems are provided. The same problem as described above arises also in the configuration where one image input system is provided for one projector system. That is, because the intensity of a pattern reflected image varies according to the view point of the image input system and the inclination of a surface of the subject, correspondence between the projected light and the reflected image captured cannot be obtained. There arises a problem that a great deal of mistaken correspondence occurs or corresponding points cannot be found.
The same problem as described above arises also in the case where an image input system having a principal point identical to that of the projector system is used. Before description of this problem, a problem arising in the case where an image input system has a principal point not identical to that of the projector system will be described. FIG. 14 is a diagram showing the configuration in which the image input system has a principal point not identical to that of the projector system. In FIG. 14, light with a plurality of stripes encoded by the color or intensity of the light is projected onto a subject 12 by the projector system 11. The striped light on the subject 12 is monitored by the image input system 15. The intensity value of the projected striped light is compared with that of the captured striped image to find the same and one stripe pattern to thereby calculate a distance according to the trigonometrical theory.
The case where the subject has a textured surface (such as a colored surface or a patterned surface) and the case where the subject has a glossy surface (exhibiting a biased intensity distribution of reflected light) will be described. First, when the subject has a textured surface, the captured striped image is affected by the textured surface of the subject. As a result, the color/brightness of the captured striped image becomes different from that of the projected striped light, so that it is difficult to judge correspondence between the captured striped image and the projected striped light. Accordingly, mistaken correspondence occurs, so that the distance to the subject cannot be calculated. To solve this problem, an image input system is provided in a position identical to the position of the principal point of the projector system 11. FIG. 15 shows this configuration. In FIG. 15, texture information is acquired by the identical principal point image input system 16. Because each of striped images captured by the identical principal point image input system 16 and the nonidentical principal point image input system 15 contains texture information of the subject, occurrence of error can be suppressed when corresponding points are extracted on the basis of comparison between the two striped images. Accordingly, the influence of texture of the subject on deterioration of measurement can be reduced. In addition, the texture information per se can be acquired accurately, so that a 3D image with texture can be acquired.
In this manner, the influence of texture can be considerably lightened by the provision of the image input system having a principal point identical to that of the projector system. As described above, it is however impossible to adapt this configuration to the case where the subject has a glossy surface.
As the related art related to the invention, there is a method in which a polarizing filter is provided between a subject and a camera to prevent unnecessary light from entering the camera when an light-section line is used for measuring the 3D shape of the subject, as disclosed in JP-A-2002-162208.